Problem: In geometry class, the girl to boy ratio is $8$ to $5$. If there are $15$ boys, how many girls are there?
Answer: A ratio of $8$ girls to $5$ boys means that there is a group of $8$ girls for every group of $5$ boys. If there are $15$ boys, then there are $3$ groups of $5$ boys each. Because we know the ratio is $8:5$ , there must be $3$ groups of $8$ girls each. There is a total of $24$ girls in geometry class.